accidental
ac-ci-den-tal
noun
A musical sharp, flat, or natural sign not in the key signature.
"The pianist played an accidental."
adjective
Happening by chance, unintentionally, or without deliberate planning.
"The accidental discovery led to a breakthrough."
Thesaurus
Synonyms: unintended, inadvertent, inessential, nonessential, unessential, incidental